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Origin of life
showing the divergence of modern species from their common ancestor in the center. The three s are colored, with blue, green, and s red.}} :See also: s reason that all living organisms on Earth must share a single , because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms. The three properties necessary for life are: :Persistence :Reproduction :Evolution Independent emergence on Earth Life on Earth is based on and . Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from . There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; , the element directly below carbon on the , does not form very many complex stable molecules, and because most of its compounds are water-insoluble, it would be more difficult for organisms to extract. The elements and have more complex chemistries, but suffer from other limitations relative to carbon. Water is an excellent and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as , are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages. Organisms based on may, however, be possible on other planets. Research on how life might have emerged from non-living chemicals focuses on three possible starting points: , an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external s, which allow food to enter and waste products to leave, but exclude unwanted substances. Research on abiogenesis still has a long way to go, since theoretical and approaches are only beginning to make contact with each other. Replication first: RNA world Even the simplest members of the of life use to record their " " and a complex array of and molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA. These s could have formed an in which there were individuals but no species, as s and s would have meant that the offspring in each generation were quite likely to have different s from those that their parents started with. RNA would later have been replaced by DNA, which is more stable and therefore can build longer genomes, expanding the range of capabilities a single organism can have. Ribozymes remain as the main components of s, modern cells' "protein factories." Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga. Although short self-replicating RNA molecules have been artificially produced in laboratories, doubts have been raised about whether natural non-biological synthesis of RNA is possible. The earliest "ribozymes" may have been formed of simpler s such as , or , which would have been replaced later by RNA. In 2003, it was proposed that porous metal sulfide would assist RNA synthesis at about and ocean-bottom pressures near s. Under this hypothesis, membranes would be the last major cell components to appear and, until then, the s would be confined to the pores. Metabolism first: Iron–sulfur world A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including and could be achieved by using and as s. Most of the steps required temperatures of about and moderate pressures, although one stage required and a pressure equivalent to that found under of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents. Membranes first: Lipid world | image=Liposome cross section.png | width=250 | height=160 | image-width = 125 | image-left=0 | image-top=0 | annotations = water-attracting heads of molecules}} water-repellent tails}} }} It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step. Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form s, double-walled "bubbles," and then reproduce themselves. Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than they would have outside. The clay hypothesis RNA is complex and there are doubts about whether it can be produced non-biologically in the wild. Some s, notably , have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their line pattern; they are subject to an analog of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules. Although this idea has not become the scientific consensus, it still has active supporters. Research in 2003 reported that montmorillonite could also accelerate the conversion of s into "bubbles," and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes. A similar hypothesis presents self-replicating iron-rich clays as the progenitors of s, lipids and s. Life "seeded" from elsewhere The Panspermia hypothesis does not explain how life arose in the first place, but simply examines the possibility of it coming from somewhere other than the Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher in the sixth century . In the twentieth century it was proposed by the , by the s and , and by and chemist . There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large impact, in which case the most credible sources are and ; by , possibly as a result of accidental by microorganisms that they brought with them; and from outside the Solar System but by natural means. Experiments in low Earth orbit, such as , demonstrated that some microorganism s can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years. Scientists are divided over the likelihood of life arising independently on Mars, or on other planets in our . 3-Aminobenzoic-acid Did life begin with or amino acids? Maybe it began with a molecule that was both a nucleic acid and an amino acid. 3-Aminobenzoic-acid: Creating the monomers in the is easy but getting the monomers to bond into a polymer is hard. So maybe it wasnt a polymer at all. Maybe it was a one dimensional liquid crystal . See . See also * * * * External links *Molecular biology of the cell Category:Life